Treat them every one as equally as possible and with respect, even those that
show little respect for you on the onset. Most will come around.
If you only have one rule you strictly enforce, make them respect the
feelings, art work and possessions of their fellow students. They might not
like each other, but have to get along.
Be prepared. Try out everything before doing it in the classroom. The time you
spend before class will be directly proportional to the enjoyment you and your
students have during the lesson.
Use humor. Don't take everything to heart, especially with the middle school
aged kids. Tease them and be prepared to get it back ten fold.
Make this your life's work -- a career, not a job, no matter how temporary the
situation. It will be so rewarding.
When things go wrong one day, the next one some students will remind you why
it's all worth it.
Get plenty of rest. Teaching is extremely taxing, but exhilarating, too.
Best of luck! Write me back if you are interested in some middle school age